Compensated asynchronous electric motor



March 25, 1930. P. LETRILLIART 1,751,484

COMPENSATED ASYNCHRONOUS ELECTRIC MOTOR Filed June 11, 1 928 1202' C544.QWMWIJE 21. 4.

Patented Mar. 25, 1930 UNl'l'D STATE5 PATEN? PIERRE LETBILLIABT, OFEARL-TS, FRANUE, ASSEGITOR TS FOR-GEE 8r. ATELIEBS DE CON- STRUCTIONSELECTRIQUES DE JEUMOET, SOCIETE ANGNYME, OF PARES, FRANCE, A

FRENCH JOINT-STOCK COMPANY COMPENSATED ASYNCHRGNOUS ELECTRIC MOTORApplication filed June 11, 1928, Serial No. 284,333, and in France Euly1, 1927.

method presents certain than that which the motor would have with itsrotor shortcircuited. The additional slip is due to the increase of theJoule losses resulting from the increased rotorcircuit resistance and tothe increase of the rotor current as soon as it is desired toovercompensate. Even for relatively low overcompensat-ion, the slip ofthe compensated motor is twice that of the short-circuited motor, whichis a serious disadvantage tor certain applications such as the drivingof compressors, blowing machines, etc, of which the yield or output isan increasing function of the speed.

The method indicated does not allow compensation in running at low load;it is dificult to arrive at a power factor of unity for a load less thanone third the normal load, with a machine simply compensated at fullload. This disadvantage is particularly noticeable in the case ofvariable load machines.

The present invention has for its object to remedy the disadvantagesabove mentioned by maintaining the phase-c mpensation constant. withinwide limits of variation of the load, without any considerable increasein the slip of the induction motor at full load, as compared with asimilar motor provided with short circuited rotor.

The invention consists in the application, instead and in place 01 theexciter usually adopted, of a non-selt-exciting machine of theseries-wound generator type, having its stator windings connectedbetween the rotor t the main motor and the brushes of its ownconnnuta-tor, the brushes being set behind the neutral axis regards thedirection of rotation by an angle less than 180 electrical degrees.

Since the cascaded machine operates a series generator upon which thereare imposed a voltage and a frequency of external origin, it possesses aresistant torque which is a function of the load on the main motor andincreases with the load. By driving the machine by means of a suitableauxiliary motor at variable speed, there can be obtained for the group acharacteristic curve of speed and power factor practically independentof the load. For this purpose, the aux 'ary motor driving the cascadedmachine, instead of being of the ordinary constant speed type, a motorof which the speed falls considerably when the load increases; forerample a series comnnuator machine or an asynchronous motor with fixedrotor resistance, if this motor is connected to an alternating currentsupply. There is no fear of excessive loss in the rotor resistancebecause the auxiliary motor is of small power.

In these conditions, the dynamic or equivalent resistance of thecascaded commutator motor being negative there is a partial compensationoi the ohmic resistances by the special eXciter. it will be understoodthat in these conditions, the slip can be reduced, without however beingever reduced to zero, for the series machine is utilized notself-exciting machine in excited working, but as a generator upon whichthere are imposed a voltage and a frequency or" external origin. it isknown that these two methods or" operation are essentially diii erent.According to the invention, it is ess atial avoid the machine working asself-excited.

Upon the accompanying drawing, there is represented diagrammatically inFigure l, method of carrying out the invention.

A is the main asynchronous motor of which the slip-rings B are connectedto the series exciter E. This eXeiter comprises a stator winding S inseries with the commutator C. it is driven by an asynchronous motor M ofwhich the slip-rings D are connected to a slip rheostat R.

Figure 2 shows the angular direction of the negative lead or shifting ofthe eXciter brushes in relation to the direction of rotation. Theoperation of the motor arranged in accordance with the invention is asfollows: The exciter, comprising a stator winding and having negativebrush lead, possesses a resistant torque which is a function of the loadon the main motor and increases with that load. The speed of theauxiliary driving motor and therefore of the exciter varies then in aninverse manner to the load on the main motor and the compensation ismaintained constant within wide limits of variation of the load, fromabout one-tenth of the load up to full load.

It can easily be shown that the slip of the main motor compensated byavariable-speed series-wound exciter in accordance with the invention,remains practically constant under varying load. This slip issubstantially equal to what would be the slip of the main motor ifprovided with snort-circuited rotor windings.

What I claim is 1. In combination with an asynchronous electric motor,an exciter formed by a series commutator machine having its brushes setit negative lead angles and being connected in cascade with the rotor ofsaid asynchronous motor, and an auxiliarymotor driving said exciterindependently of said asynchronous motor, said auxiliary motor beingadapted to fall off in speed with an increase of load.

2. In combination with an asynchronous electric motor, an exciter formedby an alternating current commutator machine having its stator windingsconnected in cascade with the rotor of said asynchronous electric motorand in series with the respective commutator brushes, said brushes beingset at angles less than 180 electrical degrees behind the neutralcommutation axis, and an auxiliary motor driving said exciterindependently of said asynchronous motor, said auxiliary motor beingadapted to fall ofi in speed with an increase of load.

3. In the art of compensated electric induction motors, aseries-characteristic cascadeconnected exciter having its brushes set atnegative lead angles and means for driving said exciter at a speed whichdecreases with an increase of load and independently of the inductionmotor.

4:. In the art of compensated electric induction motors, acascade-connected exciter having its brushes set at negative lead anglesand in eries with the respective stator coils, said exciter being drivenindependently at a speed varying in an inverse manner to the load uponthe induction motor.

5. An asynchronous electric motor having stator windings adapted to beconnected to a source of alternating current supply, and a wound rotorconnected to slip-rings, in combination with a commutator machine drivenindependently of said asynchronous motor, saidcommutator machine havingstator coils connected in cascade between the respective commutatorbrushes and the slip-rings of said rotor, and said commutator brushesbeing set at negative lead angles, and an auxiliary motor driving saidexciter, said auxiliary motor being adapted to fall ofi' in speed withan increase of load.

6. An asynchronous electric motor, having a separately driven commutatormachine connected in cascade with the rotor of the main motor for phasecompensation, characterized by the fact that the commutator machine isnon-selt-exciting and of the series-wound generator type, having itsstator windings connected to brushes set behind the neutral axis by anangle less than 180 electrical degrees and that the commutator machineis driven at variable speed by an auxiliary motor of which the speedfalls considerably when the load increases, with the object of obtainingfor the group a characteristic curve of power factor and speedpractically independent of the load.

In testimony whereof I hereunto aflix my signature.

- PIERRE LETRILLIART.

